This invention relates to a fuel cell battery using an acidic electrolyte as the electrolyte. More particularly, the present invention relates to a method of controlling the flow rate of air to be supplied to an oxidant electrode and discharging, e.g., carbon dioxide gas generated on a fuel electrode in order to suitably discharge the water generated to the oxidant electrode.
A fuel cell battery generally, as well known, consists of a fuel electrode, an oxidant electrode and an electrolyte disposed between these electrodes, and includes a fuel chamber for feeding fuel to the fuel electrode in combination with an air chamber for feeding an oxidant to the oxidant electrode. (The negative electrode is generally referred to as an "air electrode" because a gas containing oxygen, particularly air, is generally employed.)
U.S. Pat. Nos. 3,935,028, 3,992,223, 4,160,856, for example, disclose a fuel cell battery operating at a temperature near room temperature which uses an alkali electrolyte. When methanol is used as the fuel, the cell reaction can be expressed by the formulas:
At the air electrode: EQU 2O.sub.2 +4H.sub.2 O+8e.fwdarw.8OH.sup.- ( 1)
At the fuel electrode: EQU CH.sub.3 OH+8OH.sup.- .fwdarw.CO.sub.3.sup.2- +6H.sub.2 O+8e (2)
According to the cell reaction expressed as above, water is consumed on the oxidant electrode in accordance with formula (1). Hence, it is necessary to prevent the evaporation of water by limiting the flow rate of the air to the amount required as the oxidant.
Among fuel cell batteries using an acidic electrolyte (phosphoric acid), there is a hydrogen-oxygen fuel cell battery which is operated at a high temperature (190.degree. C. or above). The cell reaction of this type is expressed by the following formulas:
At the fuel electrode: EQU H.sub.2 .fwdarw.2H.sup.+ +2e (3)
At the oxidant electrode: EQU 1/2O.sub.2 +2H.sup.+ +2e.fwdarw.H.sub.2 O (4)
Accordingly, water is generated on the oxidant electrode as expressed by the formula (4).
As will be described elsewhere, however, the saturated vapor pressure of the air to be fed as the oxidant is high because the temperature is high, and the air may be fed only in amounts necessary as the oxidant.
As described above, it has been necessary conventionally to feed the air as the oxidant which is necessary for generating the power at the oxidant electrode.
If the fuel cell battery using an acidic electrolyte is operated near the normal temperature, the air required for discharging the resulting water must be fed to the oxidant electrode besides the air which is necessary as the oxidant for the power generation.
Unless the resulting water is discharged, it accumulates on the surface of the oxidant electrode and prevents air from reaching the electrode so that the oxidation reaction cannot proceed and the cell performance is reduced.
Though a fuel cell battery using an acidic electrolyte which is to be operated near the normal temperature has been examined in the past, no examination about the amount of air to be fed to the oxidant electrode has been disclosed.
On the other hand, in fuel cell batteries for direct power generation which use a liquid such as methanol or hydrazine as the fuel, carbon dioxide gas or nitrogen gas is generated as a result of the chemical reaction on the fuel electrode and the resulting gas is exhausted into a fuel chamber adjacent the fuel electrode lest the resulting gas cover the surface of the fuel electrode and inhibit the supply of the fuel to the fuel electrode. There are two known methods of discharging the resulting gas from the fuel chamber outside the battery. According to a first method, an opening is bored at the upper part of the fuel chamber. The second circulates the fuel inside the fuel chamber, or circulates a liquid mixture of the fuel and the electrolyte by means of a pump or the like disposed outside the battery. The second method also takes the resulting gas out from the fuel chamber together with the liquid mixture of the fuel and the electrolyte and discharges the mixture outside the battery from a gas separator disposed in the circulation path. In the first method, the battery cannot be used in all postures because the opening is defined at the upper part of the fuel chamber. The second method has the drawbacks that a pump is necessary for circulating the liquid mixture of the fuel and electrolyte and the separator for separating the resulting gas from the liquid mixture of the fuel and electrolyte must be disposed in the circulation system for the liquid mixture.